The present invention relates generally to radiation sensitive positive photoresist compositions and particularly to compositions containing novolak resins together with naphthoquinone diazide sensitizing agents.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in alkaline aqueous solution, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are lightly unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subject to treatment by a substrate-etching process. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which, in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, thermal stability of the image, process latitude, line width control, clean development, unexposed film loss, and the like.
Increased photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as, in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. The optimum development conditions include a constant development temperature and time in particular development mode, and a developer composition/resist composition selected to provide complete development of exposed resist areas while maintaining a maximum unexposed resist film thickness loss not exceeding about 10 percent of initial thickness.
Development contrast refers to the slope of the linear portion of the curve resulting from the plot of the log of exposure energy vs. normalized film thickness under fixed development conditions. In use, development of an exposed resist coated substrate is continued until the coating on the exposed area is substantially completely dissolved away.
Resist resolutions refers to the capability of a resist system to reproduce the smallest equally spaced line pairs on intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist. Although negative photoresists, wherein the exposed areas of resist coating become insoluble and the unexposed areas are dissolved away by the developer, have been extensively used for this purpose by the semiconductor industry, positive photoresists have inherently higher resolution and are utilized as replacements for the negative resists.
Post exposure baking, also known as diffusion baking or deferred baking, has been widely accepted in the integrated circuit industry as a means of reducing standing wave effects in positive photoresists which is an inherent problem with monochromatic exposure and reflective surfaces. The basic process involves a bake cycle after exposure, but prior to development. A typical sequence is a soft bake of 50.degree.-70.degree. C., exposure, a post exposure make up to 110.degree. C., followed by development.
It was originally suggested by E. J. Walker, IEEE Transactions on Electronic Devices, Vol. ED 22, No. 7, p. 464, July 1975 that thermal treatment of positive photoresists after exposure and prior to development may improve lithographic properties and reduce interference patterns resulting from monochromatic exposure of reflective surfaces. The mechanism proposed involved the diffusion of nonphotolized sensitizer from areas of destructive interference, where the concentration of sensitizer is at a local maximum, to areas of constructive interference, where the concentration of sensitizer is a local minimum. Locally uniform dissolution rates of photoresist in the developer were observed when temperatures for post exposure baking exceeded the temperature of soft bake. Lithographic improvements were found in working resolution and process latitude, that is the relation between line width and exposure energy. However, some undesirable changes in photospeed and slope were associated with the process as presented.
The soft bake employed by Walker was 90.degree. C. following by a 100.degree. C. post exposure bake. A modified process was later described by Dill and Shaw IBM J. Res. Develop., p 210, May 1977, and Arnold and Levison, Proceedings of Symposium Kodak Interface 1983, San Diego, Ca, p. 80, where a softbake in the 50.degree.-70.degree. C. range was followed by a 90.degree.. The lower post exposure bake essentially eliminated deleterious impact on photospeed and image slope, while the lower soft bake preserved the benefits observed by Walker.
In the process of the present invention, the post exposure baking of the positive photoresist is conducted at a much higher temperature, for example, 150.degree. C. It has been unexpectedly found that several benefits from the use of a high temperature post exposure bake emerge. These include:
Improved contrast, photolithographic process latitude and working resolution. PA1 Thermal stability of developed photoresist image at least equal to the high temperature post exposing bake temperature. PA1 Improved control of standing waves and other reflective phenomena, such as diffraction. PA1 Substantial elimination of micro-peeling and residue problems common to metal ion free developers.